Molten metals, particularly molten aluminum and steel, are frequently contaminated to some extent by entrained non-metallic inclusions that give rise to a variety of shortcomings or defects in the resulting finished products. Such inclusions can arise from a number of sources, for example, in aluminum through the entrainment of surface oxide films, from the formation of insoluble impurities such as coarse or clustered boride particles, or fine or coarse carbides and nitrides, from the oxidation of alloying elements such as magnesium, and from the erosion of the refractory linings of vessels used to hold or transport the liquid metal.
Problems that are caused by the presence of inclusions include the tearing of the metal during mechanical working operations, the presence of pin-holes and streaks in foils, surface defects and blisters in anodized sheet, and increased rates of breakage during the production of wire. These problems are becoming more acute as customers demand thinner, lighter products and better surface appearance, and as the proportion of recycled metal that is used in the production of some sheet metal products rises, with attendant increases in inclusion formation during remelting.
A variety of methods are currently available for removing inclusions from molten metals such as aluminum, and aluminum alloys including decantation, fluxing with reactive solid or gaseous mixtures (which may be combined with stirring), filtration, vacuum refining, and combinations thereof. It is also found helpful in the cleaning of liquid steel to produce turbulence in the melt, since this produces particle agglomeration and subsequent floating out of the agglomerate. The aim of these processes is to reduce the size and/or concentration of suspended inclusions to acceptably low levels. Each of these operations inevitably adds to the producer's costs.
In order to evaluate the effectiveness of these methods, producers have at their disposal a number of procedures for the evaluation of metal quality. Included in these are filtration rate tests, the metallographic examination of polished metal sections (either directly or following a preconcentration step such as filtration or centrifuging), non-destructive testing techniques such as ultra-sonic or eddy-current testing, counting the rate of defects appearing in sheet or foil, or counting the number of breakages per unit length of wire produced. Filtration rate tests can provide information relatively rapidly, but are unable to provide size distribution data. Since all the other procedures listed above entail a substantial time delay before the results are available the metal will usually be cast and possibly fabricated before the test results become available. In these circumstances the only course available to the producer when substandard metal quality is encountered is to downgrade the material, and perhaps to scrap the production lot.
Ideally, for control of commercial processes test results of liquid metal quality are needed within minutes. Additionally, the tests should provide information about the size and the concentration of any entrained inclusions. To this end the Reynolds Metal Co. has described in the Journal of Metals for October 1982 an ultrasonic, pulse-echo technique for detecting discontinuities in a sample of molten aluminum caused by unwanted inclusions. Such a system is said to be able to provide a rapid indication of the state of cleanliness of the liquid metal, but its ability to provide quantitative measurements of either the absolute concentration or the size distribution of inclusions has not as yet been demonstrated.
An apparatus for electrical zone sensing of suspended particles in a liquid is disclosed in U.S. Pat. No. 2,656,508 issued Oct. 20, 1953 to Wallace A. Coulter. In a typical apparatus a tube having an aperture in its wall is positioned within a larger vessel. A liquid electrolyte suspension containing the particles to be detected is placed in the vessel and is induced to flow through the aperture by establishing a fluid pressure differential between the interiors of the vessel and the tube. The vessel and the tube are both fabricated of an insulator, e.g. glass, and a constant electric current is placed across the aperture. The presence of a particle in the liquid flow through the aperture causes a change in the electrical resistance detected at the aperture and the electric voltage producing the constant current will vary directly with the resistance change each time a particle passes through the aperture. A detecting circuit determines the size of the passing particles from the change in resistivity caused by each particle, this depending upon the volume of electrolyte at the aperture displaced by the particle and by the resistivity of the kind of particles being sized. This information is amplified and processed by suitable electronic circuits.
This electrical sensing zone apparatus for particle size analysis is widely used in biology, haemotology, geology, mineral processing and in many industries dealing with powders. All of the applications of the electrical sensing zone apparatus described above are carried out at moderate temperature using a suspending medium consisting of aqueous or organic electrolytic solutions.